feat(desktop): embed BoosAPI Desktop client as a subproject

Vendor the rebranded BoosAPI Desktop client (Tauri + Vue) into metapi as
a monorepo subdirectory under desktop/. Source only — node_modules,
dist, Cargo target and the upstream .git history are excluded.

desktop/ keeps its own independent Cargo workspace (desktop/src-tauri
with crates/*); it is not part of the backend boosapi crate, so a
`cargo check` at the metapi root still compiles only the backend. Build
the desktop app from desktop/src-tauri per desktop/README.md.

Note: app icons are still placeholder assets pending the final BoosAPI
logo. The updater signing keys live outside the repo and are not
included.

desktop/libs/markstream-vue/test/normalize-backslash-t.test.ts

@@ -0,0 +1,23 @@
+import { normalizeStandaloneBackslashT } from 'stream-markdown-parser'
+import { describe, expect, it } from 'vitest'
+
+describe('normalizeStandaloneBackslashT', () => {
+  it('prefixes control characters with backslash', () => {
+    // simulate strings where control escapes were interpreted
+    const tab = normalizeStandaloneBackslashT('\t')
+    expect(tab).toBe('\\t')
+
+    const backspace = normalizeStandaloneBackslashT('\b')
+    expect(backspace).toBe('\\b')
+  })
+
+  it('prefixes known katex command words without backslash', () => {
+    expect(normalizeStandaloneBackslashT('in')).toBe('\\in')
+    expect(normalizeStandaloneBackslashT('perp and more')).toBe('\\perp and more')
+    expect(normalizeStandaloneBackslashT('alpha+beta')).toBe('\\alpha+\\beta')
+  })
+
+  it('escapes exclamation marks', () => {
+    expect(normalizeStandaloneBackslashT('!')).toBe('\\!')
+  })
+})